A lateral-effect position-sensing detector (LEPSD) can be part of an optical angle-of-arrival sensor system that determines the incidence angle of the SWIR or MWIR light produced (typically wavelengths of light between approximately 3 μm and 5 μm), for example, by a transient event or a rapidly moving object. In these instances, SWIR or MWIR radiation arises from heat generated or chemical reactions caused by such events or moving objects.
LEPSDs have been known with respect to detection of visible light since at least 1957, such as described in J. T. Wallmark: “A new semiconductor photocell using lateral photoeffect.” Proceedings of the IRE, Vol. 45, S. 474-483 (1957).
Prior art MWIR LEPSD elements assembled in arrays are described in B. H. Scott, R. Wolfshagen and J. Buck, “Methods for the use and manufacture of infrared position-sensing detector focal plane arrays for optical tracking,” U.S. Pat. No. 7,333,181 B1 (2008) and R. Wolfshagen, et al., “HgCdTe position sensitive detector (PSD) development,” Proceedings SPIE Vol. 7660, p. 76600H (2010). As explained in the latter reference, the LEPSD discussed therein is a quadrilateral device fabricated from HgCdTe material having a single lateral p-type current conducting layer with four contacts located on the same side of the n-type absorber layer. While the LEPSDs described in the former reference, fabricated from either InSb or HgCdTe or other IR sensitive semiconductor material, can be either the quadrilateral devices or the dual-axis devices and are said to be able to detect light of MWIR wavelengths, no dual-axis structure for such a MWIR LEPSD is disclosed or claimed.
As described in P. S. Bui and N. D. Taneja, “Position-sensing detector for the detection of light within two dimensions,” U.S. Pat. No. 6,815,790 B2 (2004), is another prior art LEPSD. This LEPSD is of the dual-axis type having an InGaAs absorber, for detecting short-wave infrared (SWIR) light, sandwiched between two layers of InP. These InP layers serve as the lateral current conducting layers and the resistive dividers with each layer having a pair of contacts. The InGaAs is a direct-band gap absorber and can be fairly thin with a thickness of 2.0-3.0 μm being typical for achieving maximal absorption of the SWIR light. The LEPSD of this reference has a PIN diode structure with high capacitance due to the thin, preferably undoped or lightly doped, InGaAs I-layer between the two InP layers, one being a P-layer and the other the N-layer. The high capacitance of this LEPSD structure can limit the response bandwidth and rise time of a large-area device.
A MWIR detector having an electron barrier in its material structure is described in P. Klipstein, “XBn barrier photodetectors for high sensitivity and high operating temperature infrared sensors,” Proceedings SPIE Vol. 6940, paper 69402U (2008) and P. Klipstein, “Depletion-less photodiode with suppressed dark current and method for producing the same,” U.S. Pat. No. 7,795,640 B2 (2010). This prior art MWIR detector has an absorber of n-type materials that is not depleted, a depleted wide band gap, electron-blocking barrier of n-type or p-type materials adjacent to the absorber, a contact layer of n-type material and a contact layer of p-type materials. However, a LEPSD formed from this structure, while desirable for a MWIR LEPSD, would have one of its lateral-current conducting layers comprise the p-type material. The inter-electrode resistance from such a hole-conducting layer would be quite high and the depleted barrier layer of this structure, having a thickness of only 0.05-1 μm, would result in a fairly high capacitance per unit area, with a concomitant degradation of response bandwidth and rise time of a large-area device. The capacitance per unit area of the photodetector increases as the thickness of the depleted region is reduced.
Another prior art MWIR detector having an electron barrier in its material structure is described in S. Maimon and G. W. Wicks, “nBn detector, an infrared detector with reduced dark current and higher operating temperature,” Applied Physics Letters, 89, 151109 (2006) and S. Maimon, “Reduced dark current photodetector,” U.S. Pat. No. 7,687,871 B2 (2010). This prior art detector has an n-type absorber, an un-doped barrier adjacent the absorber and an n-type contact layer. The thicknesses of the barrier layer is typically 0.05-0.1 μm and the contact layer is 0.02-0.05 μm. This prior art detector structure has n-doped layers (the contact layer and the substrate) that could be used for the two lateral-current conducting layers of a LEPSD. However, the photo-current through one of the contact layers is carried by the minority holes which would result in high inter-electrode resistance. Also, due to a barrier thickness of only 0.05-0.1 μm, this structure cannot provide a lateral-effect PSD with sufficiently low capacitance per unit area. Moreover, the Maimon patent does not include a broken-gap hetero-junction (herein, Type III hetero-junction) between the barrier layer and the contact layer. As a result, the problem relating to response bandwidth and rise time in the prior art is exhibited in this structure as well.
In still more prior art documents, a MWIR detector array fabricated from material that contains a barrier, absorber and collector (or contact layer) is described in D. Yap, R. D. Rajavel, S. Mehta and J. S. Colburn, “Wide bandwidth infrared detector and imager,” U.S. Pat. No. 7,928,389 B1 (2011), H. Sharifi, et al., “Fabrication of high operating temperature (HOT), visible to MWIR, nCBn photon-trap detector arrays,” Proceedings of SPIE Vol. 8704, 87041U (2013), and A. I. D'Souza, et al., “MWIR InAs1−xSbx nCBn detectors data and analysis,” Proceedings of SPIE Vol. 8353, paper 835333 (2012). For this detector array the absorber is etched into 3-dimensional pyramid-shaped structures that reduce the volume of absorber material while also trapping the incident photons in the absorber. This detector array demonstrated broadband absorption of light from below 3 μm wavelength to beyond 5 μm wavelength, at an operating temperature of 200K and reduced the dark current by approximately a factor of 3 compared to devices that were not etched. Despite its smaller absorber volume, the internal quantum efficiency of the detectors is higher than 80%. However, no LEPSD is disclosed in this prior art invention.
A prior art photodetector structure described as having an electron barrier layer that has a thickness of 3-10 μm is described in A. M. White, “Infrared detectors,” U.S. Pat. No. 4,679,063 (1987). That barrier, which blocks the flow of electrons but does not block the flow of holes, comprises p-doped material that is sandwiched, in a NPN configuration, between two layers of n-type material having smaller band gap than the material of the electron barrier. Although that prior art electron barrier is wide, only a portion of that wide barrier is depleted, with another portion of that barrier being undepleted. This photodetector structure does not have any feature to achieve depletion of the entire thickness of the electron barrier layer, nor is increase in the width of that depleted region of the electron barrier discussed as being favorable. Also, the entire NPN structure of this prior photodetector is fabricated from HgCdTe or GaAlAs (common-anion) materials and thus it cannot contain any Type III hetero-junctions.
A LEPSD with faster temporal response or wider bandwidth frequency response, than is presently available from prior art LEPSDs, is desirable because it makes possible the identification of short transient events and the tracking of more rapidly moving objects. It is also desirable to have a LEPSD with larger instantaneous field of view for a given frequency-response bandwidth, which can be achieved with a LEPSD having a smaller capacitance per unit area and thus permitting a larger light-absorbing area. A novel LEPSD with reduced dark current, low inter-electrode resistance and reduced capacitance, together with a method for making the same and a sensor system incorporating the same, is presently disclosed. This is achieved by a device that has a narrow band gap absorber, a Type III alignment of the junction between the electron barrier layer and a lateral-current conducting layer of the LEPSD and with the electron barrier designed to be >2 μm thick and to be depleted over almost its entire width or thickness.